hi,
Whenever u get this type of errors just close all the browsers and windows and restart your system, and then try to start QC again, u will be able to work on it. Perheps ot happens b'co of time out. when u have opened QC and not using it for long time. Try it and let me know whether it works or not, as I als have faced such problem, and this solution works for me .

Hi,
I understab ur problem, Whenever u open the QC site it asking for downloading the Active 'X' controls, in this it installs Test director client and all suporting instalables so u must install Active 'X' controls before u use QC.
If u got any problems after that please feel free to contact me.

4.Navigate to your windows\system32 directory and select the file
'RegSvr32.exe'. Do a file search for it if you have trouble
locating the exact folder. On Windows 2000 it is typically
located in c:\WINNT\System32. Click Open to select the file.
Then click OK on the 'Open With' dialog.

5.You should see a message indicating the file was successfully
registered. If you see an error message, try restarting your
computer and going through the above process again.